This invention relates to the field of P-ROM arrays or matrices and more particularly to a low current fusible link for programming a selected cell in such an array.
Programmable read only memories (P-ROM) are devices which can only be "written on" or "programmed" once, but can be "read" any number of times and are well known in the art. The programming process is not reversible. Early art includes magnetic cores and capacitors, both of which were expensive and slow to program. Solid state (IC) devices were developed which solved these problems to some extent, the earliest ones being programmed during the manufacturing process, which required a different diffusion mask for each program desired. The next improvement came in designing the cells on the IC chip with an element which could be selectively destroyed as desired after manufacture. In some such devices the element to be destroyed was shorted out, and in some it was opened. Resistive elements, back-to-back diodes, transistor/diode combinations, SCR/fuse combinations, Zener diodes and transistor/nichrome fuse combinations have all been used, but all require relatively large currents for programming. Frequently this large current destroyed cells not intended to be destroyed, or shorted out an element instead of opening it. Various features were designed into the devices to control the programming currents but these features required either extra processing steps or more space on the chip and thus added to the cost of the device.